1. Field of the Invention
This disclosure relates to comparators and comparing methods, particularly to comparators and comparing methods with hysteresis.
2. Description of the Related Art
A comparator is an electronic device used in comparing two signals.
FIG. 1 is a circuit diagram showing a conventional comparator. Referring to FIG. 1, the comparator includes a differential amplifier 10 and a buffer circuit 20. The differential amplifier 10 receives two input signals VIN1 and VIN2 and amplifies a difference between the two signals VIN1 and VIN2. The buffer circuit 20 includes inverters connected in cascade, and receives and buffers an output signal VAO of the differential amplifier 10. However, the comparator having the circuit configuration of FIG. 1 may not have immunity from noise when the noise is part of input with the input signals VIN1 and VIN2.
FIG. 2 is a circuit diagram showing another conventional comparator with a hysteresis circuit 32 included in the differential amplifier 30. The hysteresis circuit 32 has a latching function, and has different thresholds when the voltage level rises and when the voltage level falls because the voltage levels of nodes N1 and N2 change according to the input signals VIN1 and VIN2. The output signal VAO of the differential amplifier 30 and the output signal VOUT of the comparator may have hysteresis if the comparator has different thresholds when the voltage level rises compared to when the voltage level falls. Therefore, the noise immunity of the comparator shown in FIG. 2 may be larger than that of the comparator shown in FIG. 1. However, the operable frequency range of input signals may be decreased and the degree of the increase of the noise immunity may not be very large.